1. Field of the Invention
The invention concerns a method of processing an optical element and an optical element, in particular for a microlithographic projection exposure apparatus.
2. State of the Art
Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is known as a projection exposure apparatus having an illumination system and a projection objective. The image of a mask (=reticle) illuminated by means of the illumination system is projected by means of the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (for example photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure onto the light-sensitive coating on the substrate.
Both in the illumination system and also in the projection objective, layers or layer systems are frequently used on the optical elements, for example to achieve desired optical effects (for example as anti-reflection layers) or as protection for optical elements such as for example lenses against degradation.
It is known inter alia from DE 10 2004 059 778 A1 and US 2005/0225737 A1, in a projection objective for immersion lithography, to provide the last optical element with a protective layer system provided for contact with the immersion medium in order to increase the resistance to degradation caused by the immersion medium. The protective layer system can also include in particular a barrier layer which is substantially impermeable to the immersion medium. The disclosure of those applications is hereby incorporated to the full extent thereof by reference (“incorporation by reference”).
It is known inter alia from JP 2002022903 A to apply a water-repellent layer, for example by vapor deposition of alkoxy polymer, on a layer system applied to a lens, having an outermost SiO2 layer. It is known inter alia from JP 08164517 A for a photoelectric diode provided in a silicon substrate to be protected from washing water (used when sawing up the wafer for cleaning purposes) with a water-repellent layer. It is known inter alia from JP 11264903 A for an outer SiO2 layer to be applied to an anti-reflection layer used especially in the UV range. It is known inter alia from JP 2002014202 A for an outer layer consisting of an oxide to be formed in a layer system applied to a substrate and having a fluoride-bearing layer.
It is known inter alia from WO 00/69785 for a hydrophobic layer to be produced by thermal vapor deposition with polyfluorocarbons in a high vacuum on optical substrates which have an alkaline earth metal or alkali metal fluoride layer as the outermost layer or which comprise alkaline earth metal or alkali metal fluorides.
The problem which frequently occurs in the above-mentioned layer system (that is to say also in any protective layers which are present) is that those layer systems have pores or layer defects for example in the form of holes, cracks or “pinholes”. Defects of that kind can have an adverse effect in many respects on the imaging quality of the system and the service life of optical components of the system.
Impairment of the imaging quality can result on the one hand from the effect of form birefringence which occurs as a consequence of the presence of pores: the term “form birefringence” is used here to denote the presence of two different refractive indices by virtue of a layer structure which is present in a grown layer system when that layer structure comprises a large number of columns containing the layer material and pores therebetween. On the other hand impairment of the imaging quality can also result from filling or emptying of the pores, for example with water or hydrocarbons, which occurs repeatedly and in an undefined fashion during operation of the projection exposure apparatus.
A reduction in the length of the service life of optical components of system can result from the fact that fluorine-bearing organic substances which outgas from the photoresist are converted into gaseous hydrogen fluoride (HF) at the wavelengths used in the UV range, such as for example 193 nm, and penetrate through the pores or defects in a layer to the respective substrate behind the layer (for example the SiO2 lens) and can chemically attack it and thereby damage it. In addition, degradation for example of the last optical element, caused by an immersion medium (for example deionized water) can also occur if the immersion medium for example penetrates through layer non-homogeneities or pinholes in a protective layer and reaches the last optical element and damages it by physical processes (dissolution) or chemical processes (etching thereon).